Compact plant type rice has higher lodging and N resistance under machine transplanting

Jiangsu Collaborative Innovation Center for Modern Crop Production/National Engineering and Technology Center for Information Agriculture/Key Laboratory of Crop Physiology and Ecology in Southern China,Nanjing Agricultural University,Nanjing 210095,P.R.China

Abstract To identify the major factors that contribute to the difference in lodging among different rice varieties under machine transplanting and their responses to nitrogen (N),field experiments were conducted at Danyang County (a representative eco-site of the Lower Yangtze River) in Jiangsu Province,China in 2017 and 2018,22 hybrid indica varieties (HIs),22 inbred japonica varieties (IJs) and two indica japonica hybrid varieties (IJHs) were transplanted by machine with three N rates (N0,N150 and N300,0,150 and 300 kg ha-1,respectively). Lodging-related physical parameters,morphological characteristics and apparent transport rates of dry matter were examined. Significant difference in yield was observed among different types of rice,and followed by IJs＜HIs＜IJHs. The average lodging index (LI) of hybrid varieties (HIs and IJHs) was higher than that of the inbred varieties (IJs) with higher plant height; moreover,lower apparent export rate of dry matter resulted lower LI in IJHs than in HIs. The HIs had a large difference in the LI,which came from the difference in bending stress (BS) induced by the difference in the apparent export rate of dry matter,varieties with lower leaf angle of upper three leaves possess strong lodging resistance capacity; however,the gap among the IJs was due to the difference in the cross section modulus (Z). The LI in the IJs or IJHs increased slightly with the increased N application,and there was no lodging incidence under the high N level,which was due to the low leaf angle and barely changed under high N; there was a significant interaction between varieties (HIs) and N rates in lodging rate and LI,varieties with lower leaf angle of upper three leaves were resistant to high N. These results suggest that compact plant type rice has higher lodging and N resistance at machine-transplanting method.

Keywords:machine transplanted rice,lodging resistance,nitrogen,apparent export rate of dry matter

1.Introduction

Lodging as an important constraint factor limiting rice production in high-yielding environments has been influenced by many elements,including varieties (Islamet al.2007; Zhuet al.2016),weather (Wenget al.2017;Wuet al.2017; Zhaoet al.2019) and crop management(Zhang W Jet al.2016; Chenet al.2018; Liuet al.2018).The Yangtze River region is an important rice production base in China; disadvantageous weather conditions,such as typhoon,heavy rain and weak light are often encountered in the late growth stage of rice,which increases the necessity of growing rice with lodging resistance (Ishimaruet al.2008;Phelanet al.2010).

In recent years,Maet al.(2004) and Liet al.(2013)suggested that the potential risk of rice lodging has been increasing due to the release of high-yielding varieties with a tall plant height and large panicles. An optimal plant height with appropriate internode configurations (shorter basal internode and longer uppermost internode below the panicle) and enhanced stem stiffness could improve the lodging resistance of high yielding rice populations (Zhang Jet al.2014). In contrast,Islamet al.(2007) and Zhuet al.(2016) indicated that the lodging difference among the varieties was attributed to the difference in the culm wall diameter and thickness but not the plant height.Most studies have shown that the lodging resistance of thejaponicarice is stronger than that of theindicarice(Ookawa and Ishihara 1993; Ookawaet al.2010,2016;Hiranoet al.2014),and the QTLs for the cell wall tissues injaponicarice could be used to improve the lodging resistance inindicarice by increasing the breaking strength(Mulsantiet al.2018). These studies mostly focused on difference of stem traits among varieties,however,few studies focused on how the leaf orientation especially leaf inclination affects crop competitiveness and thus resulted in lodging. Leaf is one of the components of ideal plant type in rice,and its inclination has a great influence on yield(Chenet al.2018; Donget al.2018; Tanget al.2018). A fine leaf incliation can optimize crop population structure,improve the microclimate conditions of ventilation and light transmission in the field and also enhance the culm strength (Florenceet al.2019).

Appropriate nitrogen (N) fertilizer management is an effective and essential way to improve grain yield,while reducing lodging problems (Zhang W Jet al.2014; Wu and Ma 2019). In the high yielding region of the middle-lower Yangtze River,the average N application rate is 300 kg ha-1(Juet al.2015; Liuet al.2016). Previous studies on the response of lodging characteristics of different types of rice to N fertilizer have got a consistent result,that was,with the increased N application,the plant height increased,center of gravity moved up,base internode length increased,internode thickness decreased,stem lodging index (LI)increased,and lodging resistance decreased (Sunet al.2012; Liet al.2013; Wuet al.2015). Yanget al.(2009)showed that N reduced the stem strength and increased the lodging risk primarily by reducing the filling degree of the basal internode sheath and the content of structural carbohydrates in stem,particularly the lignin content. Most of previous studies on the effects of N fertilizer on lodging were performed with limited varieties,the results were not precise enough and informative for breeding. With this,this experiment was conducted with a large range of advanced varieties.

With the transformation of China’s economic structure and the shortage of a high-quality rural labor force,rice planting urgently needs to be simplified,mechanized and increased in scale (Zhuet al.2015). The rice planting mode in the Yangtze River region has changed from manual to mechanical (Zhang and Gong 2014). The large number of rice plants and the limited individual growth of machine transplanting generally increased the risk of lodging compared with manual transplanting,so the lodging resistance mechanism of machine-transplanted rice will be different,previous studies were performed with limited varieties (Liet al.2011; Leiet al.2013; Jiang Yet al.2014;Xinget al.2017; Liuet al.2018). However,there is a lack of systematic research on the comparison among different types of rice at machine-transplanting method. Therefore,hybridindicavarieties (HIs),inbredjaponicavarieties (IJs)andindicajaponicahybrid varieties (IJHs) popularly utilized in China since the 1990s were selected to:(1) elucidate the underlying reasons for their lodging differences by measuring lodging-related parameters under machine transplanting and (2) determine the differences of lodging characteristics of different varieties in response to N.

2.Materials and methods

2.1.Experimental site

The field experiments were conducted in 2017 and 2018 at Danyang County,Jangsu Province,China (32°00´N,119°32´E,7 m a.s.l.). This area is a part of a subtropical humid climate. The 20-cm depth of the soil below the soil surface was sampled before irrigation to measure the basic soil physicochemical properties,and the properties were as follows:organic matter of 21.09 g kg-1; the total N,total phosphorus and total potassium contents were 1.12,0.47 and 1.98 g kg-1,respectively; and the available N,available phosphorus and available potassium were 85.40,13.33 and 119.41 mg kg-1,respectively. The climate data,including daily photosynthetically active radiation and air temperature(Fig.1) were measured using a silicon pyranometer (LI-200,LI-COR Inc.,Lincoln,NE,USA) and a temperature/RH probe(HMP45C,Vaisala Inc.,Helsinki,Finland),respectively.

2.2.Experimental cultivars

Fig.1 Daily photosynthetically active radiation and daily mean temperature at Danyang County,Jiangsu Province in 2017 and 2018.

Three types of rice were used,including 22 HIs,22 IJs and two IJHs that had been primarily utilized in China during the last 30 years. The traits of all the varieties were approved by the China Rice Data Centre. The data of the testing varieties are shown in Appendices A and B.

2.3.Experimental design

The experiments were arranged in a split plot design with the N treatments as the main plots and the rice varieties as subplots,with three replications and a subplot size of 45 m2.Three N rates (N0,N150 and N300,0,150 and 300 kg ha-1,respectively) were applied as urea in three splits:40% at the basal,30% at 7 d after transplanting (DAT) and 30%at the third leaf age from the top. The P fertilizer (90 kg ha-1P2O5as single superphosphate) was applied one day before transplanting,and the K fertilizer (120 kg ha-1K2O as potassium chloride) was applied split equally at the basal and panicle initiation stages,respectively. The seeds were sown on May 30 and seedings were machine-transplanted on June 16. The hill spacing for the IJs was 12 cm×30 cm with three seedlings per hill,while the hill spacing for the HIs and IJHs cultivars were 17 cm×30 cm with one seedling per hill. Machine-transplantation was performed using the rice transplanter (PZ640,Iseki Agricultural Machinery Co.,Ltd.,Tokyo,Japan) on June 16 in both years. Banks were built around the main plots and also between different types of rice,the same type of rice was fertilized uniformly according to its growth period characteristics. Banks were wrapped with plastic film to prevent water and fertilizer leakage.Gutters were also set up between main plots to ensure single row and single irrigation. Water,weeds,insects,and disease were controlled during the entire growth stage.

2.4.Observations and measurements

At maturity,all the plants within an area of 9 m2in each plot were harvested using a Mini Half-Feeding Combine to determine the actual yield. The number of lodging plants in each plot was investigated before harvesting. The plants with the angle between the plant and the ground less than 45° were defined as lodging plants. The percentage of lodging plants in the plot was calculated as the apparent lodging rate (%).

On the 20th d after the full heading stage,10 hills with identical growth were selected and one main stem per hill was sampled to measure lodging-related physical traits,and the measuring method was followed by the measurements of Ookawaet al.(1992). A testing machine (AIKON RX-5,Tokyo,Japan) were used to push the midpoints between two supporting points with a distance of 4 cm,and the testing value was defined as the breaking load of the second elongation internode at the base. A digital vernier caliper was utilized to measure the culm morphological parameters(culm wall thickness,culm wall diameter and inner and outer cross-section). According to Ookawa and Ishimaru (1992),the physical parameter calculations were performed as follows:Whole plant bending moment (WP,g cm-1)=SL×FW,where SL is the distance from the breaking point to the panicle top (cm),and FW is the fresh plant weight from the breaking point to the panicle top (g); Breaking strength(M,g cm-1)=F×L/4,where F is the breaking load,and L is the distance between the two pivot points; Cross-section modulus (Z,mm3)=π/32×(a13b1-a23b2)/a1,where a1and a2are the outer and inner diameters of the minor axis in an oval cross-section,respectively; b1and b2are the outer and inner diameters of the major axis in an oval cross-section,respectively; and Bending stress (BS,g mm-2)=M/Z; Lodging index (LI,%)=WP/M.

At the full heading and 20th day after full heading stage,five representative hills were sampled.The samples were separated into leaves,stem sheaths and panicles,and ovendried for 30 min at 105°C to deactivate the enzymes and finally oven-dried at 85°C to a constant weight. Apparent export rate of dry weight (%)=(W1-W2)/W1,where W1is the dry weight of the stem sheath at the full heading stage,and W2is the dry weight of the stem sheath at the 20th d after full heading. The angle of upper three leaves (angle between stem and line from occipital to tip) was measured at the full heading stage.

2.5.Statistical analysis

Microsoft Excel 2016 and Origin 2019b (OriginLab Corp.,Northampton,MA,USA) were used to create the tables and figures. Data were analyzed using three-way analysis of variance with SPSS 18.0 for windows (SPSS,Inc.,Chicago,IL,USA). The means of the treatments were compared using a LSD test at a 0.05 level.

3.Results

3.1.Differences in the yield,lodging rate and Ll

The average grain yield across the three N rates and two years in the IJHs was significantly higher by 13.1 and 23.6%than that of the HIs and IJs,respectively. With increased N application,the average grain yield in the IJs and IJHs increased significantly during the two years. However,the average yield in the HIs at the N level of 300 kg ha-1was not significantly higher than that at the N level of 150 kg ha-1. Significant interactive effect between the N levels and varieties was only found in the HIs (Table 1).

No lodging occurred in the IJs and IJHs during the two years. The average value of the lodging rate of the HIs in 2017 was higher by 94.5% than in 2018,and there was a significant interaction between the N level and varieties in the HIs. The average LI in the HIs across three N levels and two years was higher by 25.6 and 47.9% than that for the IJHs and IJs,respectively. With the increase N level,the difference in LI between varieties in each type of rice all inccreased,especially the HIs which had the largest difference in LI under high N level (Table 1).

3.2.Physical parameters

There were significant differences in all physical parameters among types (Table 2; Fig.2). The average SL in the HIs or IJHs was significantly higher than that in the IJs. The average FW in the IJHs was higher by 19.7 and 44.4% than in the HIs and IJs,respectively. The average Z in the IJHs was higher by 64.2% than in the IJs but lower by 7.4% than in the HIs. The average BS in the IJs was the highest among the three types,followed by the IJHs and HIs. The FW and BS in 2018 were higher than those in 2017.

The varietal difference in the SL was not related to the LI for either type in 2017,while the correlation was significantfor the HIs in 2018 with correlation coefficient of 0.33. The varietal difference in the FW was only related to the LI for the HIs in 2018. There was a negative correlation between the BS and LI in the HIs and also between the Z and LI in the IJs in two years. When the data of the three types were pooled together,LI was only significantly correlated with SL and BS in both years.

Table 1 Grain yield and apparent lodging rate of the hybrid indica varieties (HIs),inbred japonica varieties (IJs) and indica japonica hybrid varieties (IJHs) under different nitrogen (N) levels in 2017 and 2018

3.3.Apparent export rate

Averaging across varieties and N treatments,the HIs had a higher apparent export rate than the IJs or IJHs in 2017(Fig.3),although the difference was not significant among the three types in 2018. A correlation analysis showed that the BS was negatively linearly related to the apparent export rate for the HIs in each year,while the correlation was not significant for IJs. When the data from the three types were pooled,a significant correlation was only detected in 2017.

3.4.Leaf angle of the upper three leaves

The average leaf angles of the upper three leaves for the HIs were higher than those of the IJs or IJHs. HIs also had a larger gap in leaf angle compared with IJs or IJHs. The relationship between the leaf angle with the apparent export rate is shown in Fig.4. For the HIs,the apparent export rate increased as the flag and the second leaf angle increased.The correlation was significant when the data from the three types were pooled,particularly in 2017.

3.5.N response

Fig.5 illustrates the apparent diversity at the different levels of N in the lodging-related mechanical index of the 46 varieties. There was no difference among the three types in the range of SL at different levels of N. The range of the FW and Z at different levels of N in the IJs was lower than that in the HIs or IJHs,whereas the IJs showed a higher range of BS at different levels of N than the HIs or IJHs. The range of the leaf angle at different levels of N for each leaf in the HIs was higher than that in the IJHs or IJs; the range of the flag leaf and the second leaf in the HIs increased with the leaf angle as the N0 level increased,but it barely changed in the IJs or IJHs (Fig.6).

4.Discussion

4.1.Difference in lodging among different types

In this study,there were significant differences in yield among the three types under machine transplanting,followed by IJHs＞HIs＞IJs. Analyzing mechanics index,the LIs of hybrid rice were significantly higher than those of inbred rice,which resulted from higher SL (indicative of plant height) in the hybrid varieties and higher BS in the inbred varieties; this was consistent with previous research(Jiang Y Het al.2014; Gonget al.2015). Furthermore,in the hybrid varieties,the lodging rate and LI of the IJHs decreased significantly,which was primarily attributed to an increase in the bending stress. Previous studies have generally indicated that the mechanical strength was primarily determined by the carbohydrate accumulation in the basal culms and the export of carbohydrates from the stems to kernels after heading; increased transportation will result in lower stem thickness (Panet al.2014). Our study also found the apparent transport rate of the stem sheath in the IJHs was significantly lower than that in the HIs.In our previous studies (Zhang Wet al.2016; Zhang 2017),we concluded that high non-structure carbohydrate transportation (NSC) was primarily responsible for culm carbohydrate transportation,even though more NSC transport occurred at 20 days after heading,culm strength could be increased by greater SC proportion,so increase SC accumulation at heading stage could balance therelationship between the lodging and yield.

Table 2 Analysis of variance on lodging related mechanical and morphological indexes of different types under different nitrogen(N) levels (F-value)1)

Fig.2 The relationships of the distance from the breaking point to the panicle top (SL),fresh plant weight from the breaking point to the panicle top (FW),bending stress (BS) and cross-section modulus (Z) with lodging index (LI) for the hybrid indica varieties(HIs),inbred japonica varieties (IJs) and indica japonica hybrid varieties (IJHs) at Danyang,Jiangsu Province,China in 2017 and 2018,respectively. P＜0.01 and P＜0.05,significances at the 0.01 and 0.05 probability levels according to the LSD test,respectively;P＞0.05,no significance.

Fig.3 The relationships of the apparent export rate with bending stress (BS) for the hybrid indica varieties (HIs),inbred japonica varieties (IJs) and indica japonica hybrid varieties (IJHs) at Danyang,Jiangsu Province,China in 2017 and 2018,respectively. P＜0.01 and P＜0.05,significances at the 0.01 and 0.05 probability levels according to the LSD test,respectively; P＞0.05,no significance.

However,large differences in the LI and BS were observed among the HIs,and a logarithmic positive relationship between the BS and the apparent transport rate of the stem sheath was identified in both years. These findings suggested that the lodging resistance of hybrid rice has been greatly improved (Changet al.2016) and differences in the LI of the HIs depended on the apparent transport rate of the stem sheath difference. The difference in the apparent transport rate of the stem sheath depends on whether the photosynthetic capacity of the canopy after heading could meet the needs of grain filling (Yanget al.2013; Zhanget al.2017). The difference in sink size between different types of rice was little; compared with HIs,higher photosynthetic capacity of the canopy after heading in IJs or IJHs mainly resulted from high leaf area duration during grain filling stage (Gonget al.2014; Weiet al.2016,2017). Previous studies indicated that high-yielding rice varieties,such as ‘Takanari’ with erect leaf posture,have been reported to have higher photosynthetic rates per leaf than other varieties (Taylaranet al.2011). The increase of the leaf angle in the upper leaves of the plants will cause the deterioration in the growth environment of the middle and lower leaves,decrease light transmittance in the middle and base parts of the rice population,accelerate the senescence of the basal stem sheath and increase the output rate of the stem sheath substances (Longet al.2006; Amanullahet al.2007). Our study found that the leaf angle of the flag leaf and the second leaf from the top were significantly correlated with the apparent transport rate of the stem sheath in the HIs and were significantly higher than those in the IJs or IJHs,suggesting that varieties with compact plant architecture possess a high lodging resistance. Consistent results were observed in oat (Wuet al.2019),cowpea (Wanget al.2007) and maize (Xiuet al.2017; Renet al.2019). Compared with the HIs,the LI in the inbredjaponicavarieties varied little,and the LI was significantly correlated with the modulus of the basal internode section (indicative of stem diameter and wall thickness) but not with the stem strength,indicating that the BS of thejaponicarice was currently high enough,and the LI difference in the IJs was primarily due to the difference in the stem diameter and wall thickness which was consistent with the results of Hiranoet al.(2014).

In addition,difference in lodging resistance between the two years was also observed (Table 1). Compared to 2017,the LI decreased significantly in 2018 mainly owe to higher BS. Less typhoons and rains accompanied with sufficient light and high daily temperature in the late growth stage(from heading stage to maturity stage) in 2018 resulted in an increase in the photosynthetic capacity after heading(Ookawaet al.1993),which reduced the dependence of grain filling on the transport of stored carbohydrates in the stem sheath,thereby increasing stem strength achieving the simultaneous improvement of yield and lodging resistance(Zhang Jet al.2014).

4.2.Differences in N response

Fig.4 The relationships of the apparent export rate with the flag leaf angle (FLA),the second leaf angle (SLA) and the third leaf angle (TLA) for the hybrid indica varieties (HIs),inbred japonica varieties (IJs) and indica japonica hybrid varieties (IJHs) at Danyang,Jiangsu Province,China in 2017 and 2018,respectively. P＜0.01 and P＜0.05,significances at the 0.01 and 0.05 probability levels according to the LSD test,respectively; P＞0.05,no significance.

LI responses to N among the three types were consistent.LI increased with the increase in N application. However,high N fertilizer significantly increased the lodging rate of the HIs. There were strong interactions on the LI and lodging rate between the varieties and N treatments. The effect of the mechanism of N on the lodging resistance for the different rice varieties differed. In our study,the effect of N on Z in the HIs was higher than that in IJs. In contrast,the effect of N on the BS was lower than that in the IJs,and a previous study also investigated the consistent conclusion(Zhang W Jet al.2014). Higher apparent transport rate induced by the increase in the flag leaf angle and the second leaf angle at the heading stage with high applications of N rather than lower Z was primarily responsible for the high LI in HIs under a high N level,while the leaf angles of the IJs and IJHs did not increase significantly with the increase in N fertilizer,resulting in a slightly increased LI and a lack of lodging. It was concluded that varieties with a compact plant architecture had high N tolerance and lodging resistance.According the results,we suggest that it need less tillering N but more panicle N to improve dry matter accumulate at late stage for varieties with lower leaf angle.

Fig.5 Comparison of the R-SL (the range of the distance from the breaking point to the panicle top among different nitrogen levels),R-FW (the range of fresh plant weight from the breaking point to the panicle top among different nitrogen levels),R-Z (the range of cross-section modulus among different nitrogen levels),and R-BS (the range of bending stress among different nitrogen levels) among varieties with SL-N0,FW-N0,Z-N0,and BS-N0,respectively. N0,treatment with 0 kg ha-1 applied.

Fig.6 Comparison of R-FLA (the range of flag leaf angle among different nitrogen levels),R-SLA (the range of second leaf angle among different nitrogen levels) and R-TLA (the range of third leaf angle among different nitrogen levels) among different FLA-N0,SLA-N0 and TLA-N0,respectively. N0,treatment with 0 kg ha-1 applied. P＜0.01 and P＜0.05,significances at the 0.01 and 0.05 probability levels according to the LSD test,respectively; P＞0.05,no significance.

5.Conclusion

The LI in the hybrid rice varieties was higher than that in the inbred rice varieties with a higher plant height. Moreover,compared to the HIs,the IJHs have a substantial genetic improvement in the apparent transport rate of dry matter,resulting in higher lodging resistance. The large difference in the HIs was attributed to the difference in the apparent transport rate of dry matter,and the difference in the IJs could be explained by the difference in the stem diameter.The IJs,IJHs and HIs (varieties with lower leaf angle of upper three leaves) have a high lodging resistance capacity and were resistant to high N levels.

Acknowledgements

Funding was provided by the National Natural Science Foundation of China (31871573),the National Key Research and Development Program of China (2016YFD0300505,2017YFD0301200 and 2018YFD0300803) and the Jiangsu Key Research and Development Program,China(BE2017369).

Appendicesassociated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm